The need for accurately measuring the weight of material being transported on a belt conveyor has long been recognized. Knowing the weight of the transported material is particularly important when granulated commodities are involved, such as grain, ore, aggregate, minerals, and coal because certain variables, such as belt speed and the flow rate of the conveyed material, may need to be adjusted for optimal conveying. Weigh feeders are commonly used to regulate the flow of, for example, powdered coal into a coal furnace used at an electric power generating plant.
A primary problem with traditional conveyor scales is their restricted range of application. Conveyors come in a variety of sizes and different load ratings. Materials as diverse as heavy mineral ores, on the one hand, and as light as bags of cotton or tea leaves, on the other hand, may need to be conveyed. Conveyor belts of many widths and load ratings are manufactured for all conceivable situations. This has traditionally required a large number of different conveyor scales.
Attempts have been made to overcome the problems of traditional belt scales by mounting one or more strain sensors on the top surface of the cantilevered portion that supports the idler assembly. The strain sensors measure the stress in the cantilevered portion to measure the weight of material being transported. Such arrangements have also had problems with inaccurate weight measurements because the strain gages lie in a different plane than the carry roll (which supports most of the weight of the material being transported) of the idler assembly. The weight of material being conveyed causes friction when the belt strikes the idler roller and places a horizontal force on the roller. This creates a torque effect or an overturning moment which affects the measurement readings of the strain sensors and thus the accuracy of the weight measurement, since the strain sensors ideally will measure only the stress due to the weight of material on the belt.
Certain prior belt scale mounting arrangements have been designed to avoid interference between the weigh beams and the ends of the idler assembly and the stringers that support the idler assembly. Such mounting arrangements have traditionally involved crossbars. Some of these arrangements have resulted in the strain sensors mounted on the cantilevered weigh beam being placed closer to the level at which the material is being conveyed. Such prior weigh scales have, however, failed to appreciate the importance of carefully adjusting the position of the weigh beam so that the sensors are located at approximately the same horizontal level as the material being weighed.
An additional problem with traditional belt scales is that the conveyor truss or framework experiences shifting, twisting, or movement because of material loading, temperature changes, and vibration from heavy process machinery. Because traditional conveyor scales include a rigid crossbar coupled between the stringers to which a pair of weigh beams are connected, movement of the conveyor frame by the above-mentioned forces cause the weigh idlers on the scale to shift and move, which adversely affects the weight signal.
A problem in common with the above-described traditional weighing mechanisms for conveyor belts is that a specific weigh beam or weighing mechanism must be tailored for each belt conveyor system and the particular weight of material to be conveyed. Conveyor belts come in many different widths and thicknesses according to the type of hauling required. Hence, many types of conveyor scales, such as weigh beams, have traditionally have been required to accommodate the various conveying systems. Factors that have traditionally been unique to the specific designs include the cross-sectional size, the strength, and the length of the cantilever portion of the weigh beam. The costs associated with constructing and inventorying these multiple weighing mechanisms are substantial.